Montgomery County Moves to Expand ADU Rights—Public Hearing Set for June 8

The proposed amendment makes four major updates:

Where ADUs Are Allowed

Under current county code, ADUs are only permitted in Agricultural, Rural Residential, and Residential Detached zones. ZTA 26-06 would open ADU eligibility to every zone that allows a single-family detached dwelling—including townhouse, multi-family, and most commercial/residential mixed-use zones—essentially expanding by-right ADU access across nearly the entire county. Only industrial zones would remain excluded.

How ADU Size Is Calculated

The current code applies separate size caps to attached ADUs (1,200 sq. ft. maximum) and detached ADUs (the lesser of 50% of the principal home’s footprint, 10% of the lot, or 1,200 sq. ft.). ZTA 26-06 replaces both standards with a single, state-defined rule: an ADU may not exceed 75% of the size of the primary dwelling.

Setback Requirements for Detached ADUs

Existing rules require detached ADUs to meet the same side setbacks as the principal structure, a fixed 12-foot rear setback, and an escalating setback formula for structures wider than 24 feet. The proposed amendment eliminates these stricter standards and instead simply requires detached ADUs to meet the same setbacks that apply to any other accessory structure—generally more permissive.

Density and Parking

ADUs would be excluded from density calculations in all zones countywide (currently the exclusion only applies in Agricultural and Rural Residential zones). On parking, the county conducted a required parking study and elected to retain its existing one-space-per-ADU requirement, but with two notable modifications: properties that currently have no on-site parking would no longer be required to provide two spaces when adding an ADU—just one—and a new Hearing Examiner waiver would allow properties where lot size or shape makes parking physically impractical to seek an exemption.